An optical fiber collimator having a GRIN lens (Graded Index Lens) fused to the tip end of an optical fiber can connect a semiconductor laser and an optical fiber with high efficiency or provide a connector with few connection loss, for example, and thus is useful as various optical-communication components.
As methods for manufacturing such a GRIN lens, an ion exchange method, a vapor phase CVD method, and the like are mentioned and it is said that a sol-gel method using a low-temperature synthesis method as a base is excellent. A method for manufacturing a GRIN lens by a sol-gel method is disclosed in Patent Documents 1 and 2 described later, for example. The method includes adding an acid or a base as a solvent to an alcohol solution containing a silicon alkoxide (Si(OR)4 (R: alkyl group)) as the main ingredients and hydrolyzing to form a sol, and further subjecting the sol to a polycondensation reaction and maturing to thereby progress a crosslinking reaction to form a wet gel. The production of a GRIN lens requires forming a concentration distribution in a dopant (metal ingredients imparting a refractive-index distribution). The refractive index becomes high in a region where the dopant concentration is high. Thus, the concentration of the central portion of a GRIN lens is made high, so that the concentration becomes lower toward the outside. A method using a metal alkoxide or a metal salt as raw materials of the dopant or a molecule stuffing method is mentioned, and the use of alkoxides of Ti, Ta, Sb, or Zr is particularly useful. In order to form a concentration distribution, leaching is generally performed. In the leaching, a wet gel is immersed in an acid solution, and a dopant around the periphery is eluted to give a concentration distribution. Then, the obtained wet gel is dried to remove the solvent in the gel followed by sintering, thereby producing a dense cylindrical-glass base material to which a refractive index distribution is given. Then, by thinly drawing the glass base material, a GRIN lens is manufactured.
A former sol-gel method includes forming a base material using two ingredients of silicon and a dopant (e.g., SiO2—TiO2 when the dopant is Ti) and has a problem in that foaming is likely to occur during sintering and drawing and the yield becomes poor. The foaming during sintering and drawing greatly relates to the bulk density of a dry gel. More specifically, when the bulk density is high (when the porosity is low), organic materials or moisture contained in the dry gel cannot be completely removed during sintering for producing the base material and cause defects, such as cracking or foaming, due to the heat history during sintering and drawing. Therefore, it is important how the bulk density can be reduced and controlled to a desired level.
Hitherto, an attempt of controlling the bulk density by controlling a chloride concentration, a leaching time, a leaching temperature, or the like during leaching has been made. However, it is impossible to reduce the bulk density while forming a desired refractive-index distribution, in other words, to simultaneously achieve two effects. Thus, a priority is given to imparting a favorable refractive index distribution, which inevitably results in a problem in that the bulk density becomes high.
The following Patent Document 3 has proposed, in order to make the bulk density of a base material small, a technique including producing a wet gel using three ingredients of silicon, a dopant, and aluminum, and eluting the aluminum from the wet gel by leaching to make the bulk density small.
Thus, in the leaching step, almost all the aluminum elutes from the wet gel to increase the porosity. A dry gel obtained by drying the same also has a high porosity. When the dry gel having a high porosity is sintered, the gas in the gel escapes from the pores to the outside, and thus a glass base material in which gas is hardly present is produced. Therefore, cracking due to expansion of the gas in the gel does not occur and foaming does not occur even when the base material is drawn, and thus the yield during sintering and drawing is sharply improved.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2005-115097    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2005-145751    [Patent Document 3] Japanese Patent No. 4084838